This
simple
circuit
is sure
to have
the
police
beating
a path
to your
door -
however,
it has
the
added
advantage
of
alerting
you to
their
presence
even
before
their
footsteps
fall on
the
doormat.
The
circuit
transmits
on
Medium
Wave
(this is
the
small
problem
with the
police).
IC1a,
together
with a
sensor
(try a
20cm x
20cm
sheet of
tin
foil)
oscillates
at just
over
1MHz.
This is
modulated
by an
audio
frequency
(a
continuous
beep)
produced
by IC1b.
When a
hand or
a foot
approaches
the
sensor,
the
frequency
of the
transmitter
(IC1a)
drops
appreciably.
Suppose
now that
the
circuit
transmits
at 1MHz.
Suppose
also
that
your
radio is
tuned to
a
frequency
just
below
this.
The 1MHz
transmission
will
therefore
not be
heard by
the
radio.
But
bring a
hand or
a foot
near to
the
sensor,
and the
transmitter's
frequency
will
drop,
and a
beep
will be
heard
from the
radio.
Attach
the
antenna
to a
multiplug
adapter
that is
plugged
into the
mains,
and you
will
find
that the
Medium
Wave
transmission
radiates
from
every
wire in
your
house.
Now
place a
suitably
tuned
Medium
Wave
radio
near
some
wires or
a plug
point in
your
house,
and an
early-warning
system
is set
up.
Instead
of using
the
sheet of
tin foil
as the
sensor,
you
could
use a
doorknob,
or
burglar
bars. Or
you
could
use a
pushbutton
and
series
resistor
(wired
in
series
with the
33K
resistor
- the
pushbutton
would
short it
out) to
decrease
the
frequency
of IC1a,
so
activating
the
system
by means
of a
pushbutton
switch.
In this
case,
the
radio
would be
tuned to
a
frequency
just
below
that of
the
transmitter.
